High performance switching and computation digital electrical systems such as telephone central office switches and centralized computer systems oftentimes require large cable wiring harnesses to interconnect circuit boards, backplanes and frames containing electronic processors and circuitry comprising elements of the systems. Other distributed computer systems, such as work stations and parallel processing computers, need to transmit information between processors, disk drives and other peripherals at data rates capable of supporting transmission of uncompressed high resolution video and data transfer to and from mass storage devices, color printers and displays.
To support operation of these systems, data transmission over distances ranging from 1 to 100 or more meters with a channel data rate of at least 50 Mb/s is essential. Various types of transmission media including cable harnesses of wire conductors, ribbon cables having multiple parallel metallic conductors, optical glass fibers and coaxial cables and transmission lines have been used to interconnect system circuit boards, backplane and frames. Typically, large cable harnesses of wire conductors have been used to interconnect circuit boards, backplanes and frames of telephone switching and computer mainframe systems. For short distances less than 1 meter, flexible ribbon cables having multiple parallel conductors are commonly used inside disk drives and in computer cabinets. Optical and coaxial cables have multiple and single optical fibers and coaxial conductors each commonly used to transmit independent strings of digital signals. Coaxial transmission lines have been used on circuit boards to interconnect circuit board components and to transmit digital signals from one component to another. Strip line cables have also been used to transmit microwave and millimeter wave signals. Typically, such strip line cables have conductors embedded in a dielectric material having a ground plane above and below and which conductors are coupled with driver and receiver circuits. High density and high performance flexible cables using conductors embedded in an insulating material having shields located on one or both sides of the insulating material have been proposed for digital applications.
A problem arises in that loss characteristics of cable harness conductors make them unsuitable for high frequency operations needed with modern and future high speed switching and computer systems. Another problem arises in ribbon cables having multiple metallic conductors and optical fibers and in the independent optical fibers and coaxial conductors of optical and coaxial cables in controlling both the loss and skew of transmitting digital signals on parallel conductors and optical fibers. Yet another problem arises in using present circuit board coaxial transmission lines due to both the loss and skew characteristics inherent in long lengths of transmission lines required to interconnect high speed computer and switching systems. Other problems arise with strip line cables having conductors tightly coupled through a shield in that differential signals appearing on conductors are effectively cancelled thereby making these type of transmission lines unsuitable for differential signal transmission. Problems also arise with present flexible cables in that conductors embedded within an insulating material located between shields makes it difficult to control both the cable characteristic impedance and crosstalk between conductors. Accordingly, a need exists for a flexible thin film multi-conductor transmission line capable of transmitting parallel digital data over distances ranging from 1 to 100 or more meters with a channel data rate of at least 50 Mb/s with a minimum coupling between conductors and a protecting shield and a minimum of crosstalk while still maintaining a flexible transmission line.